The answer is proximity. The Gestalt principle of proximity explains why people tend to see objects that are near each other as a unit.
Gestalt principles of visual perception theorize how our brains percieve the world; psychologists in the 1920's discovered that we process images by creating structures and patterns. They explained this process using seven principles, also known as the seven Gestalt principlesof visual perception.
The principle of proximity explains why we see objects that are close together as if they were related, when they are near each other we tend to group them without taking into account other characteristics such as shape or color.